Cells and batteries based on metals such as those having an alkali metal-containing anode (negative electrode) and compatible cathode (positive electrode) are known. Particularly favored are such cells comprising a lithium foil anode active material of a thickness of about 75 microns, or a composite intercalation anode layer of the same thickness. Also favored is an intercalation cathode layer of a thickness similar to the anode and which contains finely divided transition metal oxide active material, electrically conductive carbon, and a solid electrolyte material. An electrolyte layer having a thickness of about 25 microns is positioned between the anode and the cathode and often comprises an ion conducting polymer such as polyethylene oxide complexed with an ionizable alkali metal salt. The electrolyte layer separates the anode and cathode from one another while providing transport of ions between the anode and cathode. Typically, a current collector of conductive metal is positioned on the sides of both of the electrodes away from the electrolyte layer.
Processes for making electrochemical cells containing alkali metal active material and components of such cells are generally described in U.S. Pat. No. 5,011,501 to Shackle and U.S. Pat. No. 4,935,317 to Fauteau and Shackle. Each of these patents are incorporated in its entirety herein.
Lithium based cells or batteries are particularly favored and often comprise cathodes of the aforementioned transition metal intercalation compounds. The intercalation reaction involves the interstitial introduction of a guest species, namely, lithium into the host lattice of the transition metal oxide, essentially without structural modification of the host lattice. Such intercalation is essentially reversible because suitable transition states are achieved for both the forward and reverse of the intercalation action. The basic components of a lithium cell typically include a lithium containing anode, a separator, and a metal oxide intercalation cathode active material such as vanadium oxide also referred to as vanadates or vanadate compounds. The cathode is usually a mixture of such oxide compounds and other aforementioned components such as graphite and an electrolyte binder which provide electronic and ionic transport. During cell operation, incorporation of lithium in the metal oxide occurs. Some vanadates have high initial capacities, which, however, rapidly decline especially in the first cycles. Many metal oxides are prepared in a complex process by mixing precursor components containing an alkali metal with vanadium pentoxide and then baking the mixture to a sinter or melt temperature in the range of about 600.degree. C. to 800.degree. C. to cause formation of the product. The high temperature product of this process is then cooled and ground up into a powder. Such high temperature melt and sintering processes have certain disadvantages because it is difficult to handle molten or sintered metal oxides at high temperatures and special procedures are required. In addition, there is a reaction between the molten product and the containers used for conducting the reaction which thereby causes contamination of the product; and a significant amount of mechanical energy is required to grind the cooled solidified products to form a powder for inclusion in a cathode composition of an electrochemical cell. Despite these difficulties, high temperature melt or sintering processes such as described in U.S. Pat. No. 5,013,620 continue to be used to obtain positive electrode active material, such as LiV.sub.3 O.sub.8 . Recently, it has been suggested to form vanadium oxide compounds by reaction of a precursor oxide with an alkali hydroxide such as LiOH (U.S. Pat. No. 5,039,582 to Pistoia). Still despite the many available compounds and methods, it is desirable to have a new active material which has a high specific energy, high cycle life, and high rate capabilities; and a method for preparing such active material which is relatively simple and economical, which does not require handling metal oxide constituents in a high temperature sintered or molten state, and which achieves good conversion of the starting materials to the final metal oxide product.
The present invention provides cathode active materials having as their major component or consisting essentially entirely of an oxide of vanadium of one of the nominal general formulas: Li.sub.a Fe.sub.x V.sub.y O.sub.z and Li.sub.m Fe.sub.x V.sub.y O.sub.z. In the case of Li.sub.a Fe.sub.x V.sub.y O.sub.z the material in an initial condition does not contain any lithium so a is equal to 0. In the case of Li.sub.m Fe.sub.x V.sub.y O.sub.z the material in an initial condition contains lithium so m is greater than 0. In both cases, x is about 1, y is about 3, and z is about 8 corresponding to a trivanadate MV.sub.3 O.sub.8 unit structure. Importantly, the active material is prepared with at least a portion of the vanadium in the V(4) state. This means at least a portion of the vanadium is in the plus 4 (+4) state.
In an as prepared form or initial first condition the Li.sub.a Fe.sub.x V.sub.y O.sub.z contains no lithium; when electrochemically reduced, fully or partially discharged to a second condition the amount of lithium increases to a value of up to about 4 so that a varies between a is equal to 0 and a is less than or equal to 4 (0=a.ltoreq.4).
In the case of the material of the formula Li.sub.m Fe.sub.x V.sub.y O.sub.z, in an initial first condition as prepared, it contains some lithium so that m equals m1 and m1 is greater than 0; when electrochemically reduced, fully or partially discharged to a second condition, m equals m2 and m2 is greater than m1 and is up to about 4; and when electrochemically oxidized, fully or partially charged to a third condition, m equals m3 and m3 is less than m1 . In an initial first condition as prepared m1 is close to or about equal to 1. In the oxidized fully or partially charged condition m3 is close to or about equal to 0. Preferably in said respective conditions m1 is 1 and m3 is 0. Importantly, it is possible to insert up to about 4 equivalent atoms of Li per equivalent unit of V.sub.3.
The active material of the invention provides surprising capacity increase under certain conditions. In an initial as prepared condition, the material has the constituents in the atomic ratios presented. However, the lithium initially present in the material can be removed by charging to 4.25 V either after the cell has been discharged or before cycling of the cell. In this way, the capacity increases upon further cycling and capacity is increased to at least 4 Li per V.sub.3. Most cells show a slightly increasing capacity during the first 5-10 cycles. The upper limit to the lithium insertion is not known but it is greater than 4 Li per V.sub.3 and may be as great as 4.5 Li per V.sub.3 and may be less than 5. In order to avoid confusion, the reference character "n" will be used in place of "m" to describe the state of the active material. Accordingly, Li.sub.m Fe.sub.x V.sub.y O.sub.z and Li.sub.n Fe.sub.x V.sub.y O.sub.z are the same in an as prepared condition. In an initial condition n is n1 and n1 is greater than 0; in a charge to remove Li ( i. e., 4.25 V) n is n2 and n2 is less than n1 ; in a subsequent discharge n is n3 and n3 is greater than n1 . Upon further charge and discharge the value of n3 increases, and it may be greater than 4 Li per V.sub.3 unit.
Preferably, the vanadium oxide based active materials of the invention (Li.sub.a Fe.sub.x V.sub.y O.sub.z, Li.sub.m Fe.sub.x V.sub.y O.sub.z) are prepared for use in cells with an anode active material made of lithium or a compound or alloy which includes lithium. The cells also include an electrolyte which is electrochemically stable with respect to the cathode active material and the lithium, and which allows lithium ions from the anode (negative electrode) to move through the electrolyte to react electrochemically with the cathode (positive electrode) active material of the invention. The electrolyte may be liquid, solid, polymeric and in the case of a liquid electrolyte, typically includes a separator. A preferred lithium cell comprises the positive electrode active material of the invention, a negative electrode which is metallic lithium, and an electrolyte which is in the form of a polymeric network containing an electrolyte solution comprising a metal salt of lithium.
In one embodiment, the cathode active material of the nominal general formula Li.sub.a Fe.sub.x V.sub.y O.sub.z with a equal to 0 corresponding to Fe.sub.x V.sub.y O.sub.z is prepared in a series of steps. In the first step, a mixture comprising a liquid; metallic iron particles; and vanadium pentoxide containing vanadium in the plus 5 (+5) state is prepared with the relative amounts of 1 mole of iron for every 1 1/2 moles of the vanadium pentoxide. The metallic iron is reacted with the vanadium pentoxide in the presence of oxygen to change at least a portion of the V(5) to a V(4) state and to form a greenish/black gel containing the oxide of vanadium in the V(4) state. By this means, reduction of V(5) with metallic iron in solution to V(4) is achieved. In the next step of the process the liquid is separated from the mixture containing the oxide-based gel to provide a solid material comprising iron, vanadium, and oxide in a crystal structure corresponding to a trivanadate and having at least a portion of the vanadium in the V(4) state. Further characterization of the product reveals that depending on the preparation method the solid material product comprises the oxide of the crystal structure described above having at least a portion of the vanadium in the V(4) state and one or more of particles of iron and particles of vanadium pentoxide may also be present. It is preferred that all of the iron and V.sub.2 O.sub.5 be reacted so that there are no free particles of either in the product.
It is preferred that the reaction step be conducted in a range of up to about the boiling point of the mixture or liquid in the mixture and preferably no less than about room temperature (i.e., 10.degree. C.). The reaction may be conducted in a range of 50.degree. C. to 90.degree. C. with about 80.degree. C. being preferred.
It is preferred that the liquid in the mixture be water and that the step of separating the liquid from the oxide solid material be conducted by freeze-drying by cooling the water to a temperature below its freezing point under subatmospheric pressure for a time sufficient to remove at least a portion if not essentially all of the water. In order to remove virtually all of the water it is preferred that the step of freeze-drying be followed by calcining where the calcining is conducted at a temperature of at least about 200.degree. C. It is preferred that the source of oxygen for the reaction be from water or air. It should be noted that the relative proportions of constituents in the final product may be slightly different from the V.sub.3 O.sub.8 values. Although the relative proportions of Fe to V to O in the final product are nominally 1 to 3 to 8, chemical analysis of the product formed by the method described above revealed Fe.sub.0.99 V.sub.3 O.sub.8.16. Accordingly, the value of oxygen ranges between about 7.8 and 8.2 and the value of iron is somewhat less than 1 and maybe somewhat greater than 1 within about the same range of variation as for the aforesaid oxygen.
In another embodiment of the invention an iron containing vanadium oxide of the nominal general formula Li.sub.m Fe.sub.x V.sub.y O.sub.z, is prepared having a proportion of Li to Fe to V to O of approximately 1 to 1 to 3 to 8. Importantly, the product contains at least a portion of the vanadium in the V(4) state. The product showed some ferromagnetism, therefore, the solid product is thought to be a ferrite vanadium oxide product or a mixture of constituents, including LiV.sub.3 O.sub.8 having at least a portion of the vanadium in the plus 4 state and metallic iron. This product was prepared according to the general procedure described hereinabove with the additional step of introducing into the mixture lithium from the hydroxide while at the same time taking care to prevent re-oxidation of the vanadium by conducting the process under an inert atmosphere. The inert gas is a gas which is inert with respect to the components of the mixture and does not react with components of the mixture. A suitable gas is nitrogen, argon, helium, and the like. The formation of this product began with preparing a mixture comprising a liquid; metallic iron particles; vanadium pentoxide containing vanadium in the V(5) state; and lithium hydroxide. In this mixture, the metallic iron, the vanadium pentoxide, and the lithium hydroxide are reacted to change at least a portion of the V(5) to a V(4) state and to provide a gel containing the oxide of vanadium having vanadium in the V(4) state. Next, the liquid is separated from the oxide based gel to provide a solid material comprising iron, lithium, vanadium, and oxygen and having a crystal structure corresponding to a trivanadate structure with at least a portion of the vanadium in the V(4) state. It was determined that the reaction was complete when a blackish gel was produced and little or virtually no metallic particles of iron were observable. The gel was freeze dried and calcined as described above.
On calcining, the product becomes ferromagnetic indicating the presence of free iron in the final product.
The vanadium oxide products of the invention are generally in the form of porous lumps which are easily friable to a powder having surprisingly small particle size on the order of 1 micron. The products were tested in a cell to determine the behavior of specific capacity during charge and discharge and showed markedly improved characteristics as compared to conventionally known vanadates.
It is an object of the invention to provide a new method for preparing metal oxide positive electrode active materials for a lithium or alkali metal battery. Another object is to provide a lithium or alkali metal battery having good charge/discharge capacity. Another object is to provide an improved electrochemical battery based on lithium or an alkali metal which maintains its integrity over prolonged life cycle as compared to presently used batteries. Another object is to provide a vanadate based active material which is relatively cheap, easy to prepare, with a high specific energy, high life cycle, and high rate capability. Another object is to provide good conversion of the starting materials to the metal oxide products. These and other objects, features, and advantages will become apparent from the following description of the preferred embodiments, claims, and accompanying drawings.